A colony of dogs, heterozygous with respect to the gene for cyclic hematopoiesis, will be maintained. Homozygote offspring will be used to study: 1) platelet function and the levels of thrombopoietin in serums and plasmas; 2) the erythroid and granulocytic responses in vitro of marrow cultures to erythropoietin and other stimulating agents; and 3) the morphology of the marrow at all stages of the cycle of hematopoiesis. BIBLIOGRAPHIC REFERENCES: Jones, J.B.; R.D. Lange; T.J. Yang; H. Vodopick; and E.S. Jones: Canine Cyclic Neutropenia: Erythropoietin and Platelet Cycles After Bone Marrow Transplantation. Blood, 45: 213; 1975; Jones, J.B.; R.D. Lange; and E.S. Jones: Cyclic Hematopoiesis in a Colony of Dogs. J. Am. Vet. Med. Assn., 166: 365; 1975.